1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine, a facsimile, or a complex machine of these apparatuses, and, more particularly to a tandem color image forming apparatus and a color image forming method.
2. Description of the Related Art
Among color image forming apparatuses such as a color printer and a color copying machine, there is a color image forming apparatus including four kinds of toners in total, i.e., a Y (yellow) toner, an M (magenta) toner, a C (cyan) toner, and a K (black) toner, which is used for, for example, printing of characters and the like, and an image transfer engine in which developing devices and drum-like photoconductive members corresponding to the toners are united as units to sequentially transfer images using transfer units of the respective units. The color image forming apparatus forms toner images on the respective photoconductive members and transfers the toner images onto a print sheet via a transfer belt as an intermediate transfer member or directly transfers the toner images onto the print sheet. The color image forming apparatus has a color printing mode for performing printing using these four kinds of toners and a monochrome printing mode for performing black and white printing using only one kind of toner, the K toner.
As a printing system in the color printing mode, there is a quadruple tandem system. In the quadruple tandem system, toner supplying devices, developing devices, and photoconductive members corresponding to the four kinds of toners, respectively, are united as process units and the respective process units are provided side by side on a sheet conveying path. High-speed print processing is realized by forming toner images of the respective colors on the respective photoconductive members and, then, sequentially transferring the respective toner images onto one piece of transfer paper in one process.
In the process units, deterioration due to friction and the like between the photoconductive members and the intermediate transfer belt occurs and wear between the rotating photoconductive members and the developing devices occurs. Thus, in the color image forming apparatus that employs the quadruple tandem system, in general, during the monochrome printing mode, unnecessary wear of the process units is prevented by setting the process unit for K in contact with the intermediate transfer belt and separating the other process units for Y, M, and C not in use from the intermediate transfer belt and the like to prevent other process units from operating.
However, in a print job in which monochrome image data and color image data are mixed, an image forming operation of the color image forming apparatus is stopped or suspended every time the color image forming apparatus performs an operation for switching the printing modes. Thus, as the number of times of switching of the printing mode increases, time until the print job is finished increases to deteriorate productivity (throughput) of printing.
As described above, in the quadruple tandem color image forming apparatus in the past, when it is attempted to improve the throughput of printing, wear of the process units is caused. Conversely, when the switching of the printing mode is performed by a mechanical method in an attempt to reduce wear of the process units, the throughput falls.
To cope with this problem, JP-A-2001-121788 proposes a color image forming apparatus that has the following function. The monochrome printing mode is set as default of a printing mode (a predefined or initial state of a printing mode adopted when a user does not set the printing mode). Although it is possible to switch the monochrome printing mode to the color printing mode during the execution of one print job, basically, the color image forming apparatus prohibits, once the monochrome printing mode is switched to the color printing mode, switching from the color printing mode to the monochrome printing mode until the print job is completed and allows, when a predetermined number of monochrome printed pages continue or when jobs for plural copies are designated in one print job, switching from the color printing mode to the monochrome printing mode in a break between specified numbers of the copies.
However, in the color image forming apparatus in the past, since the monochrome printing mode is set as the default of the printing mode, when a user mixes a color image in printing of a small number of images such as three images, it is necessary to switch the monochrome printing mode to the color printing mode. Thus, it takes a certain time to print only a few images. The color image forming apparatus cannot start printing until it is determined whether image data of a first image includes only monochrome print data or the image data includes color print data. As a result, the print start speed is low. For example, when the user attempts to print plural copies of image data of two images, first one of which is a monochrome image and a second one of which is a color image, the color image forming apparatus alternately prints monochrome image data and color image data and switches the printing mode to the default monochrome printing mode in breaks between the numbers of the copies. Thus, switching of the printing mode is performed every time a copy is printed. As a result, throughput of the printing considerably falls.